The long term objective of this study is to contribute to the better understanding of the basic mechanisms involved in mammalian fertilization and consequently to the better control of human fertility. In this study, major emphasis will be placed on analyzing the processes and mechanisms of sperm-egg fusion and associated phenomena. Hamsters, mice and guinea pigs will be used as model animals. Specific questions we intend to answer include: 1. When does the egg plasma membrane gain and lose the ability to fuse with spermatozoa? 2. What sort of chemical changes make acrosome-reacted spermatozoa fusible? 3. What sort of membrane molecules are involved in sperm-egg fusion? 4. Can acrosome-reacted spermatozoa fuse with any type of (artificial) membranes? 5. What sort of agents or conditions prevent or promote sperm-egg fusion? 6. Is it possible to render living spermatozoa permanently incapable of fusing with eggs? 7. Is it possible to fuse isolated sperm nuclei with eggs? 8. Are calpain and intracellular calcium ions involved in the decondensation of the sperm nucleus in the cytoplasm of a fertilized egg? 9. How stable are isolated sperm nuclei in vitro? 10. Can an isolated sperm nucleus participate in the development of an embryo after being injected into an egg?